The invention relates to furnaces, especially carbottom furnaces, wherein a car for supporting articles to be heat treated, is movable along a trackway into the furnace and provides the bottom of the heat treatment chamber of the furnace that normally uses excess air burners. Such furnaces are operated in temperature ranges of, for example, 700.degree. F.-1800.degree. F. and require excessive amounts of heated air in the heat treatment process, but such excess air creates problems. For example, a great deal of energy is required to heat such excess air.
U.S. Pat. No. 4,083,677 recognizes the problems of using excessive amounts of air, and replaces the normally required large number of excess air burners with substantially fewer high velocity or high momentum-type burners that are operated in complete stoichiometric ratio of combustion air to fuel, to optimize fuel efficiency. High momentum burners are capable of providing the equivalent circulation of large quantities of excess hot gas necessary in the aforementioned heat treatment process, until the burners are turned down as the temperatures of the articles, to be treated, approach the desired level. The '677 patent solves this loss of air during burner turndown by circulating excess air into the heat treatment chamber through perforations in the burner block. It can be appreciated that this particular system is still energy inefficient because the high momentum burners become, in essence during turndown, excess air burners wherein energy is still required to heat the excess air.
This invention is designed to produce a more energy efficient system by utilizing the normally wasted hot exhaust gas, created during the heat treatment process, as a source of heated excess gas that is supplied to the chamber during high fire and turndown of the high momentum burners.
Briefly stated, the invention is in a furnace that has a heat treatment chamber that is sealable from the ambient atmosphere. Means are provided for transporting workpieces or articles, to be heat treated, into and out of the chamber. A plurality of fuel efficient high velocity or high momentum-type stoichiometric burners are mounted on the furnace to circulate hot gaseous products of combustion into the chamber in a desired flow pattern to produce the most uniform gas temperatures within the chamber. Means are provided for removing hot exhaust gas from the chamber and recirculating it immediately, without reheating, back to the chamber in a way that compliments and reinforces the flow of hot gases of combustion in the chamber, to optimize the heat treatment of the articles placed within the chamber, especially during turndown of the burners. Also, there is a substantial reduction in the fuel required, because of the efficient use of the recirculated exhaust gas.